The present invention relates to an optical system for microscopes which makes it possible to make the objectives of different microscope systems interchangeable.
The known microscopes at present available on the market operate in accordance with different, incompatible systems which differ
(a) in the exit vertex focal length of the exchangeable objectives used and
(b) in the state of correction of the interchangeable objectives.
Heretofore it has been generally customary to use objectives of finite vertex focal length in microscopes. These objectives produce directly an intermediate image of the object which is observed by means of the eyepiece. Objective and eyepiece are a defined distance apart, which distance is determined by the position of the intermediate image, i.e., in microscopes with objectives of infinite vertex focal length the mechanical tube length is a given size.
In general, in the case of microscope objectives of finite vertex focal length (referred to hereinafter as objectives of the first type) the transverse chromatic aberration is not fully corrected. The degree of correction differs from manufacturer to manufacturer. The transverse chromatic aberration which is still present in the intermediate image is compensated for by special eyepieces.
During the last few years microscope objectives with infinite vertex focal length have been developed. In them there is a parallel ray path behind the objective so that it is readily possible to introduce supplementary optics, such as beam splitters, filters, etc. In the case of microscope objectives of infinite vertex focal length (hereinafter referred to as objectives of the second type) the intermediate image is produced by an additional lens system, the so-called tube lens system.
It is clear that in microscope objectives of the second type the mechanical tube length is not a defined size; it can be of different size depending on the requirements.
Microscope objectives of infinite vertex focal length which are so corrected that an aberration-free intermediate image is produced in combination with the tube-lens system are also known.
If now a manufacturer of microscopes having objectives of the first type would like to change to microscopes having objectives of the second type, this would be very expensive for him to do, because for a rather long period of transition he would have to take into account the desire of the users for the possibility of expanding their old instruments, and therefore the manufacturer must for a long time offer components of both systems. Even then, however, the problem arises for the user that upon transfer to microscopes having objectives of the second type, his previous interchangeable objectives cannot be used, due to the lack of compatibility.
It is known from U.S. Pat. No. 3,132,200 to combine non-fully corrected microscope objectives which have infinite exit vertex focal length with a correction member which is installed fixed in the tube and produces an aberration-free intermediate image. The correction member however, due to its monochromatic aberrations, has an optimum corrective effect only for one interchangeable objective of the specific series of objectives. The introduction of a positive coma by the correction member results in a positive astigmatism at this place. Both aberrations are particularly disadvantageous in case of displacements of the pupil such as occur upon focusing movements between objective and tube lens and upon the introduction of beam splitters or filters.
From German Auslegeschrift (Provisional Patent) No. 1,472,082 it is known to use a fixed correction member, in the form of a negative cemented component, which increases the vertex focal length of interchangeable objectives and compensates for field curvature and astigmatism. Its 5.times. magnifying action is adapted to the use of a special series of objectives of low magnifying power.
In German Pat. No. 2,047,673 there is described also a fixed correction member which, with a correcting action similar to the one proposed in the above mentioned Auslegeschrift No. 1,472,082, shifts the exit vertex focal length of the interchangeable lenses to infinity. A fixed positive single lens, also provided in the tube, corrects the transverse chromatic aberration of the lenses and produces an aberration-free intermediate image. This correction member also changes the linear magnification of the interchangeable objectives.
In all of these known correction systems the correction member is integrated in fixed position in the tube and thus establishes the type of series of objectives to be used. Re-equipping of the system by installing other correction systems is time-consuming and tedious, and thus expensive. These correction systems permit compatibility of different systems therefore only in the sense that new series of objectives can be used by a single re-equipping of older tubes.